Ill try short and to the point.
2 Years ago replace front pump. It quit working.
Replace FPR, filter, FP relay.
Almost all ignition .
New computer.
This week replace rear fuel pump. I put dielectric grease on connection. Was this a mistake?
Pump worked for about a mile. Friend was driving for me. He switched to front pump it all of a sudden is working.
Bought and installed new Dorman fuel check valves.
Back pump acts like it wants to start, but doesn't. It also shows no gas. sending unit was working before.Pump is activating like it should when key is on. Have not tested fuel pressure since new back pump and check valves.

 

Maybe your dash switch is bad.

 

Does anyone know what wires are what on the switch?
It seems if I remember there are about 6.

I did not see in original post, it is a
91 F150 xlt lariat 5L dual tanks. not sure an the tranny

 

I found a diagram!

Does anyone think that the dielectric grease could be the culprit?

 

Could be increasing the resistance at the pins and not letting the connection be made.

 

Cleaned up connection, just in case. No change. May be a ground issue. Shouldn't all metal parts of vehicle be a ground source? Probing black wire at pump and ohming out to frame, I have no reading. I would think I should Have something. If not to frame at least to tank. So it would seem. If not where should I look for a ground?

Selector switch checks good. Would I only get a ground with key on?

Went to double check ground. Got continuity between pump ground and sender ground, and between those to the tank. I think this confirms ground.

Again pump seems to be activating. Why did the sending unit all of a sudden just quit working after replacing pump? It can not, I do not think be a ground because the pump is activating.
It must be a faulty pump. ???

I left original check valves in place on both pumps. Front is working back is not.Therefore I do not believe it to be a check valve issue.

 

Just finished pulling tank back down. No flow from tank. Tank operates.

Lesson learned, and most likely will never need this knowledge again.
In less than 2 miles, the back pump depleted every bit of 2 gallons of fuel. These check valves!!! If the tank sounds like it is operating, based on my own experiences I would say splice in a new check valve as close to the tanks as possible and then if this does not correct issue then start replacing other cheaper fuel related parts, of course after checking with pressure gauge.

This is a starting issue!

I may or may not have a leaking injector(s), but I am not sure how much they will affect starting..

 

Glad you got it running

 

If you happen to be someone that has to install the Dorman check valve part #800-195 (I think I referenced a wrong part # before 800-196) do your self a favor and get the permatex fuel resistant sealant part#85420.

The valve comes with two fittings on it that appear to be tight, however they leak. As I said do your self a favor and take them off and put the sealant on then you should have no problems. The valve instructions are pretty vague. There is no mention of having to do anything with these fittings.

If you happen to be doing this to the rear pump, if you do not already know there is an outer sleeve around the actual fuel line. You will want to rim off about 1/2 to 3/4 off an inch.

 

I'm pretty sure I remember someone in here say at some point he went through 3 aftermarket fuel pumps before he finally got a motorcraft fuel pump and it has worked ever since.

If I have to replace a fuel pump I will buy motorcraft for sure. I've heard way too many negative things about aftermarket fuel pumps for our trucks.